1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the amount of electric charge on a finely divided powder such as, for example, liquid crystal spacers interposed between the glass sheet of a liquid crystal substrate, which constitutes a liquid crystal display panel of a liquid crystal display device and so on, and another glass sheet, when the finely divided powder is transported using a carrier medium gas having a very small water content (low humidity) and a dew-point of, for example, 0.degree. C. or less as well as a finely divided powder spraying method and apparatus for uniformly spraying the finely divided powder in a strictly controlled amount using the above method and apparatus.
2. Description of the Related Art
At present, the liquid crystal panel of liquid crystal display devices and so on is arranged such that a trace amount of liquid crystal spacers having a particle size of several microns are interposed between the glass sheet of a liquid crystal substrate and another glass sheet for holding the glass sheet. These liquid crystal spacers are formed in a single layer in a discrete state and the amount thereof is up to several thousands of particles, such as, for example, 10 to 2000 particles in a unit area of 1 mm.sup.2. Various plastic particles and silica particles are used as the liquid crystal spacers.
For the above purpose, there is used a liquid crystal spacer spraying apparatus for uniformly spraying a prescribed amount of the liquid crystal particles onto the glass sheet of the liquid crystal substrate in a single layer.
As the liquid crystal spacer spraying apparatus, there have been used spraying apparatuses for uniformly spraying liquid crystal spacers onto a glass substrate by suspending the liquid crystal spacers in a liquid of chlorofluorocarbons or the like in a colloidal state, uniformly spraying the spacers onto the glass sheet in a liquid state and vaporizing the liquid of chlorofluorocarbons or the like. However, the spraying apparatuses using chlorofluorocarbons or the like cannot be used because they are restricted or prohibited due to the problem of environmental pollution.
To cope with the above circumstances, there have been proposed liquid crystal spacer spraying apparatuses using a gas such as air, nitrogen gas and so on in place of chlorofluorocarbons. These liquid crystal spacer spraying apparatuses transport fine liquid crystal spacer particles together with a gas flow through a thin pipe (transportation pipe) and spray them onto a glass sheet by means of a swing nozzle. The liquid crystal spacer particles are a finely divided powder that has a particle size of about several microns and is liable to float. Further, since the liquid crystal spacer particles are composed of various plastic particles or silica particles, they are liable to be charged and it is difficult to spray them onto a glass sheet at a prescribed density with excellent reproducibility. To solve this problem, the glass sheet is charged to a polarity which is opposite to the polarity (electrostatic polarity) of the liquid crystal spacer particles being charged to permit the liquid crystal spacer particles to be uniformly sprayed onto the glass sheet at a given density with reliability.
As a method of improving the reliability and reproducibility when the liquid crystal spacer particles are sprayed onto a glass sheet as well as improving the accuracy of the density of them at which they are sprayed, there is employed a method of actively charging the liquid crystal spacer particles. However, a phenomenon of charging is relatively poor in reproducibility and, in particular, even if it is intended to measure the amount of electric charge of a powder and a finely divided powder, the result of the measurement greatly fluctuates. Accordingly, this method has a limit in the improvement of reproducibility of spraying and accuracy of spraying density.
Another method intends to improve the reliability and reproducibility of spraying and the accuracy of spraying density by limiting the amount of electric charge of liquid crystal spacer particles only to the amount of electric charge which inevitably generated by the collision of the spacer particles against the inner wall of a transportation pipe and making the amount of the thus generated electric charge constant by setting constant prescribed transporting and spraying conditions for the spacer particles. The applicant has put a liquid crystal spacer spraying apparatus "Model DISPA-R" in market as a liquid crystal spacer spraying apparatus employing the above method.
The latter method of charging the liquid crystal spacer particles only passively to make the amount of the inevitably generated electric charge constant can permit the liquid crystal spacer particles to be sprayed at a prescribed spraying density at a pinpoint accuracy with excellent reproducibility, when the prescribed transporting condition is set within a transportation pipe for transporting the liquid crystal spacer particles and the prescribed spraying condition is set for the spray nozzle. However, this method has a problem that the number of liquid crystal spacer particles which will be sprayed onto a glass substrate is varied, an intended spray cannot be executed and the liquid crystal spacer particles cannot be sprayed at all in an extreme case, when there is a change in the type of the liquid crystal spacer particles, the piping system such as the transportation pipe, the spray nozzle and so on, the type and state of a gas serving as a carrier medium and environmental conditions such as an atmosphere and so on or when any conditions change while the spacer particles are continuously sprayed.
An object of the present invention is to solve the problems of the prior art and provide a method and apparatus capable of controlling the amount of electric charge inevitably generated on a finely divided powder, even if there is a change in the type of the finely divided powder such as liquid crystal spacers and so on, the piping system such as a powder transportation pipe, the spray nozzle and so on, the type and state of a gas serving as a carrier medium and environmental conditions such as an atmosphere and so on as well as to provide a finely divided powder spraying method and apparatus capable of stably spraying the finely divided powder at a prescribed target spraying density with pinpoint accuracy having excellent reproducibility by controlling the amount of electric charge on the finely divided powder.